Method for controlling the grind in a single stage autogenous grinding mill

ABSTRACT

Crushed run-of-the-mine ore is fed to an autogenous mill in size ranges arbitrarily classified as relatively coarse (RCF), relatively medium (RMF) and relatively fine (RFF) particles. The output from the autogenous mill is also arbitrarily classified as relatively coarse (RCO), relatively medium (RMO) and relatively fine (RFO) particles. The particles in the output are smaller than the crushed run-of-the-mine ore fed to the mill. The relatively coarse particles (RCO) of the output are stored, and, intermittently, amounts thereof are returned to the mill as a portion of the feed thereto to maintain a ratio of the size consist in the mill at 35/40 percent relatively coarse:20/30 percent relatively medium:35/40 percent relatively fine particles. The relatively medium particles (RMO) can be recycled to the mill as a part of the feed. The relatively fine (RFO) particles in the output from the mill are passed to beneficiation. Maximum throughput of run-of-the-mine ore at constant power input to the mill is maintained.

United States Patent Funk METHOD FOR CONTROLLING THE GRIND IN A SINGLESTAGE AUTOGENOUS GRINDING MILL I HOPPER I Feb. 6, 1973 57 ABSTRACTCrushed run-of-themine ore is fed to an autogenous mill in size rangesarbitrarily classified as relatively coarse (RCF), relatively medium(RMF) and relatively fine (RFF) particles. The output from theautogenous mill is also arbitrarily classified as relatively coarse(RCO), relatively medium (RMO) and relatively fine (RFO) particles. Theparticles in the output are smaller than the crushed run-of-the-mine orefed to the mill. The relatively coarse particles (RCO) of the output arestored, and, intermittently, amounts thereof are returned to the mill asa portion of the feed thereto to maintain a ratio of the size consist inthe mill at 35/40 percent relatively coarse:20/3O percent relativelymediumz35/4O percent relatively fine particles. The relatively mediumparticles (RMO) can be recycled to the mill as a part of the feed. Therelatively fine (RFO) particles in the output from the mill are passedto beneficiation. Maximum throughput of run-of-the-mine ore at constantpower input to the mill is maintained.

1 Claim, 2 Drawing Figures RECYCLED R C O y FEED MIX AUTOGENOUS MILL \ IMlLLP I 2a OUT UT RCO PARTICLE l3 STORAGE l4 SCREENS TO SEPARATORPATENTEDFEBHB 1975 3,715,083

HOPPER} RECYCLED RCO AUTOGENOUS FEED MIX M|LL u l mu.

m OUTPUT RCO E" PARTICLE STORAGE TD SEPARATOR -1 HOPPER RECYCLED R c oAUTOGENOUS FEED mx mu. u

MILL

UTP 'r I20 RCO PARTICLE V STORAGE RECYCLED PARTICLES RMO '4 SCREENS R F0H TO SEPARATOR INVENTOR Robert/l4 Funk METHOD FOR CONTROLLING THE GRINDIN A SINGLE STAGE AUTOGENOUS GRINDING MILL BACKGROUND OF THE INVENTIONtrolling the size consist therein.

To prepare the run-of-the-mine ore for autogenous grinding, the largepieces of raw ore obtained in mining are crushed to a range of sizesarbitrarily referred to as relatively coarse, relatively medium andrelatively fine particles which are fed to the mill. These particleswhich are fed to the mill hereinafter will be referred to as relativelycoarse feed (RCF), relatively medium feed (RMF) and relatively fine feed(RFF). Generally, the above size ranges are set at the primary crusher.However, it is impossible to control the amounts of material in eachsize range. The autogenous mill produces its own grind of particleshereinafter referred to as relatively coarse output (RCO), relativelymedium output (RMO) and relatively tine output (RFO). The size of theRFC particlesis less than the size of the RFF particles. The size of theRMO particles is most comparable in size to the REF particles, and thesize of the RC particles is most comparable in size to the RMFparticles. As is well known, a portion of the ore which is ground andpassed out of the mill contains particles which are too large to bebeneficiated and a portion'of the discharge is fine enough to be passedto beneficiation. The particles which are too large for beneficiationare recycled to the mill as part of the feed. The recycled materialcontains all the particles which are classified as RC0 and RMO particlesin the output of the mill. If a major portion of the recycled materialconsists of RMO particles, the amount of the RCF particles and RMFparticles in the size consist in the mill is reduced. There is aninsufficient amount of RCF particles in the mill for effective grinding.The mill becomes overloaded, necessitating either a cut-back in theamount of run-of-the-mine ore feed to the mill and/or an increase in thepower consumption to the mill to maintain production. On the other hand,if the recycled material contains a major portion of RCO particles thereis an attendant buildup of this size particle in the size consist in themill. The mill becomes overloaded with this size particle and a cut-backin the amount of run-ofthemine ore feed to the mill and/or an increasein the power to the mill is required to maintain production. In

SUMMARY OF THE INVENTION Broadly, the method of the invention comprisescon- I trolling the amount of the particles having a critical size rangerecycled to the mill from the discharge therefrom, to control the sizeconsist in the mill.

DESCRIPTION OF THE DRAWING FIG. I is a schematic view of the: autogenousgrinding 0 mill circuit of the invention.

either case, production is reduced and power is increased therebycausing an increase in costs and a decrease in efficiency in operation.

It is therefore an object of this invention to provide a method forcontrolling the size consist in the autogenous mill to control thegrinding of a raw friable run-of-the-mine ore in theautogenous grindingmill FIG. 2 is a branch circuit added to the circuit of FIG. 1 wherebythe method of the invention can be practiced.

PREFERRED EMBODIMENT OF THE INVENTION In the practice of the invention,FIG. 1, a raw friable run-of-the-mine ore is crushed in a primarycrusher (not shown) and is stored in hopper 10. The primary crusher (notshown) produces crushed raw friable runof-the-mine ore which containsparticles covering a wide scope of particles having sizes which arearbitrarily classified as relatively coarse (RCF), relatively medium(RMF) and relatively fine (RFF). The amounts of each size range from theprimary crusher are not readily controllable. Therefore, the crushed rawfriable ore delivered to the autogenous mill 12 from storage hopper bythe conveyor 11 contains varying amounts of the three size ranges. Thesize ranges can be +6 inch, 6,+3 inch and 3 inch, respectively. I havemaximum throughput of ore at constant power, the size consist of theparticles in the mill should be in a weight ratio of about 35 percent toabout 40 percent particles having a size +6 inch, about percent to aboutpercent particles having a size 6,1+3 inch and about to about percentparticles having a size 3 inch. Deviations from the desired ratiomentioned above result in either reduced production of the mill atconstant power input or increased power input to the mill to maintainproduction. I have found that by con trolling the amount of materialfrom the output of the mill recycled to the feed of the mill, the sizeconsist in the mill can be controlled to contain the above mentionedweight ratio. The particles 13 in the output from the autogenous mill 12are also classified into three size ranges designated as relativelycoarse (RCO), relatively medium (RMO) and relatively fine (RFO). Thelargest size particles in the output from the mill are controlled by ascreen 12a having apertures which will allow all particles smaller thana predetermined size to pass through as the output of the mill. I preferto use a screen having apertures which will allow particles having asize of up to about -3 inches to pass out of the mill. The particlesizes in the aforementioned output size ranges are fixed by screen 14having appropriate apertures therein. I prefer to classify the particlesize of the output from the mill into the following size ranges:

relatively coarse (RCO) +l inch, relatively medium I (RMO) l,+ 56 inchand relativelyfine (RFO) inch. The size consist of the output from themill is as follows:

Output from Particle Size Size Autogenous Mill ,Coarse +l inch 7.5%Medium l inch 27.5% Fine -96 inch 75.0%

I have found that the RC particles are a critical size. The RCOparticles are stored in hopper 15. The amount of the RC0 particlesrecycled to the mill is,

therefore, easily controlled thereby controlling the size that is, ifthe ratio of particles varies from the aforementioned ratio because ofthe lack of RCO particles, reduction in the mill will be essentially byabrasion rather than by attrition or impact. A material build-up in themill occurs because reduction by abrasion is a slower and moreinefficient method of reducing particle size than is attrition orimpact. The result is a net increase in grinding power requirement dueto lower feed at the same mill power. To reestablish the ratio of thesize consist in the mill, a portion of the RC0 particles in the milloutput which has been stored is charged into the mill as a portion ofthe feed thereto. The amount of the critical sized particles in the sizeconsist in the mill is increased. The amounts of the other sizeparticles in the size consist are decreased and the above desired weightratio in the size consist in the mill is reestablished. Maximumthroughput at constant power is now obtained.

If all the RC0 particles, +1 inch, from the output are recycled to themill and an imbalance in the size consist in the autogenous mill iscreated because the amount of RMF particles therein has increased tomore than about 30 percent by weight, the throughput of the autogenousmill is again reduced. The millwill continue to produce an overabundanceof RCO particles in the output therefrom. A build-up of this size rangein the mill occurs. Power consumption must be increased to maintainmaximum throughput. To correct the im- 7 balance, the portion of the RC0particles from the mill output usually recycled as part of the feed tothe mill is discontinued. The RCO particles are diverted to a secondarycrusher to be reduced in size and passed to beneficiation. The amount ofthe critical RCO particles in the size consist in the mill is therebyreduced. The desired weight ratio of particles in the size consist isreestablished. The throughput of the mill is reestablished and maximumthroughput at constant power is obtained.

In an example of the invention, it was desired to grind 0.6 ton oftaconite ore per hour in a pilot plant autogenous grinding mill. Thefeed to the mill was as follows:

Particle Size in Size Consist in Mill RCF +6 inches 40% RMF 6,+3 inches20% RFF 3 inches 40% The mill discharge was as follows:

Particle Size Dischar e from Autogenous Mi 1 RC0 +1 inch 11.3% RMO 1%inch 18.5% inch 15.4% RFO -46 inch 54.8%

The desired particle size for beneficiation was 200 mesh. The milloperated at about 13.4 KW hr./LT. of new feed. Increasing the amount ofRCF particles in the size consist in the mill resulted in an increase ofthe inch particles in the output. The throughput of the mill wasdecreased to 0.51 tons per hour. The power to the mill was increased to14.0 KW hr./LT. of new feed to increase throughput. In order to maintainthe desired ratio in the size consist in the mill about 0.10 ton perhour of+1 inch size particles (RCO) from the output of the mill wasrecycled to the feed to the mill. The desired weight ratio of particleswas reestablished in the mill. Throughput in the mill increased to 0.58tons per hour and the mill power was 13.5 KW hr./LT. of new feed.

Adding an increased amount of RMF particles to increase an amountthereof in the size consist in the mill to more than 20 percent resultedin a drop in the throughput of the mill to 0.52 ton per hour, the powerto the mill was increased to 13.8 KW hr./LT. of new feed. The amount ofthe RC0 particles in the mill output which was recycled to the mill wasdecreased to an amount which reduced the amount of this particle size inthe mill to about 20 percent. Throughput of the mill increased 0.58 tonper hour. The power to the mill was reduced to 13.5 KW. hr./LT. of newfeed.

In this specification and claims wherever percentages or ratios arereferred to, such percentages and ratios are by weight unless otherwisenoted.

lclaim:

1. An improvement in the method of autogenous grinding run-of-the-mineore in which the feed to said mill is ore having relatively coarseparticles, relatively medium particles and relatively fine particles,the ore discharged from said mill is classified into relatively coarseparticles, relatively medium particles and relatively fine particles,and the relatively fine particles discharged from said mill are passedto beneficiation, the improvement comprising passing that portion of theore discharged from said mill and classified as relatively coarseparticles to storage and intermittently feeding a portion of therelatively coarse particles from storage to said mill to control theratio of the amount of relatively coarse particles, relatively mediumparticles and relatively fine particles in the size consist in the mill.

